(December 2009) The U.S. population is projected to increase over the next four decades, but according to new supplemental projections from the U.S. Census Bureau, the rate of increase depends largely on future trends in international migration. The U.S. population currently stands at 308 million, but could increase to 399 million, 423 million, or 458 million by 2050 depending on immigration trends over the next 40 years.

The Census Bureau projections are based on three immigration scenarios, or “variants,” each with different implications for growth:

Low immigration: Assumes that net international migration will range from 1.1 million to 1.8 million per year.

Constant immigration: Assumes immigration levels will remain at current levels (just under 1 million per year).

High immigration: Assumes immigration levels will range from 1.5 million to 2.4 million per year.

A fourth scenario—produced for analytical purposes only—shows how the U.S. population would change if net immigration dropped to zero. Even if immigration came to a halt, the U.S. population would continue to increase, reaching 323 million by 2050. This growth in the absence of new immigrants reflects the young age structure of the U.S. population (relative to other developed countries), which creates population momentum through the large number of births relative to deaths. In 2008, the United States registered 4.2 million births but only 2.4 million deaths—resulting in the addition of 1.8 million people through natural increase. In fact, natural increase is estimated to have accounted for two-thirds of U.S. population growth during the past few years. Under this scenario, population aging would eventually result in a gradual decline in the U.S. population (starting in 2048), as the number of deaths starts to exceed the number of births.

Future immigration levels are hard to predict because they depend on a complex mix of “push” and “pull” factors in sending and receiving countries. Historically, the United States has been attractive to immigrants because of its strong economy and demand for both low-skilled and high-skilled workers. The availability of work in agriculture, construction, and manufacturing has attracted millions of low-skilled workers from Latin America, especially Mexico. However, immigration levels have dropped since the onset of the recession, mostly due to a decline in new arrivals rather than immigrants returning to their home countries. Future immigration trends depend on the availability of jobs as well as changes in federal and state immigration laws.

Immigration to the United States is also affected by “push” factors in sending countries. Economic development and demographic changes in Latin America or Asia could reduce immigration to the United States if labor conditions and wages improve in those regions. Mexicans account for nearly a third (32 percent) of foreigners who have entered the United States since 2000. However, the fertility rate in Mexico has dropped from over six births per woman in 1970 to fewer than three births per woman today. If fertility rates remain low, there will be a reduced supply of Mexican workers to emigrate to the United States. Over the long term, Mexico’s ability to retain its young work force also hinges on the availability of good jobs with decent wages within Mexico.

Because immigrants have characteristics that are different from the U.S.-born population, they are also changing the country’s racial and ethnic composition. Population projections released last year showed that the United States could reach majority-minority status—the point at which less than half of the population is non-Hispanic white—by 2042. However, future trends in immigration will play a key role in determining when we reach the majority-minority threshold. The latest projections show that majority-minority status could be reached as early as 2040 (based on a high-immigration scenario), or as late as 2050 (assuming immigration remains constant).

Under the zero-immigration model, the share of non-Hispanic whites would continue to decline each year, but would still exceed the 50-percent threshold in 2050. In 2008, whites made up about 66 percent of the population, and this would drop to 58 percent by 2050 in the absence of any international migration.

The new projections also show the effect of immigration on population aging, but differences in the projected U.S. age structure are relatively small. Under the low, high, and constant models, older Americans are projected to make up between 20 percent and 21 percent of the U.S. population by 2050. The zero-migration model is the only one that projects more rapid aging of the U.S. population, with the elderly accounting for 24 percent of the population in 2050. Regardless of future immigration levels, baby boomers—who have started reaching retirement age—are going to contribute to rapid population aging in the coming decades.

Mark Mather is associate vice president, Domestic Programs, at the Population Reference Bureau.